Leading article: It is time our politicians began taking bird flu seriously

Forget the threat of biological weapons, the terrorist use of smallpox or the re-emergence of a host of other frightening diseases; what really keeps world health experts awake at night is the prospect of an influenza pandemic. In particular they worry about a strain of bird flu called H5N1 which is both capable of infecting people and highly lethal. More than half of those unfortunate enough to have developed serious symptoms have died.

Compare its lethality to the flu virus behind the worst pandemic in modern human history, the outbreak of "Spanish flu" in the winter of 1918-19 when, according to conservative estimates, 40 million people died. The strain of the influenza virus behind that global epidemic is believed to have killed only a few per cent of the people it infected. A virus that kills 50 per cent would be far, far worse.

The world now is far more densely populated and urbanised than it was in 1918. Global travel makes it possible for flu viruses to spread rapidly from one part of the globe to another. These viruses are also able to mutate into more infectious forms. All this makes the threat of a truly awful pandemic very real. No wonder officials at the World Health Organisation are having nightmares.

What is preventing a pandemic at the moment is the fact that the avian flu virus has not yet mutated into a form easily transmitted from one person to another. Almost all the 112 or so people - most living in Vietnam and Thailand - who have been hospitalised with the virus have caught it from close contact with birds, usually ducks, geese or chickens. Until the virus learns the trick of person-to-person transmission, it cannot cause a pandemic.

But such viral trickery is only a matter of time. The H5N1 strain has probably infected millions of birds, both domestic poultry and wild species. It has also been found to infect pigs - the classic "mixing vessels" where different flu viruses swap genetic material to generate even deadlier strains. The more animals the virus infects, the greater the chances of it mutating into a form that could be easily transmitted between people.

This is why we should be taking avian flu seriously. The appearance of the virus in Central Asia and Siberia shows how readily it can be spread by migrating birds. Obviously, migrating birds infected with a deadly form of avian flu are not going to fly very far. But some species may be relatively immune to the virus, and so able to carry it over great distances.

In Britain, no species of migratory waterfowl that winter here come from the areas of Russia currently affected by avian flu. But should the virus manage to cross to the European side of the Ural mountains, there is then a strong possibility that it could infect the migratory species that visit Britain. The discovery yesterday of a gull in Finland thought to be infected with a non-deadly strain of bird flu suggests this could well be only a matter of time.

This, of course, would be a major threat to free-range poultry kept outside, where they can easily come into contact with wild birds. As an animal health issue, it would pose a major problem, but with the right sort of biosecurity measures, it would not be insurmountable.

What we should really be concerned about are the human health consequences of avian flu. The wider the geographical range of the deadly virus, the greater the risk it poses to humans. We are, warns the WHO, closer now to a pandemic than at any time since 1968, when the last of the previous century's three pandemics began. This is a nightmare that our politicians, not just our scientists, should be losing sleep over.